(a) Field of the Invention
The present invention relates to a display panel and a driving method thereof. More specifically, the present invention relates to an organic electroluminescent (EL) display panel and a driving method thereof.
(b) Description of the Related Art
In general, an organic electroluminescent (EL) display electrically excites a phosphorous organic compound to emit light. A light emitting pixel includes an anode (ITO), an organic thin film, and a cathode layer (metal). The organic thin film has a multi-layer structure including an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL) for maintaining balance between electrons and holes and improving emitting efficiencies. Further, the organic emitting cell includes an electron injecting layer (EIL) and a hole injecting layer (HIL).
The organic emitting cells are arranged in an N×M matrix format to configure an organic EL display panel which displays image data by voltage- or current-driving.
FIG. 1 shows a brief diagram of a general organic EL display. Exemplary embodiments of the present invention can be applied to the EL display of FIG. 1, as well as other suitable displays.
As shown, the organic EL display includes an organic EL display panel 10, a scan driver 20, and a data driver 30.
The organic EL display panel 10 includes a plurality of data lines D1 to DM arranged in the row direction, a plurality of scan lines S1 to SN arranged in the column direction, and a plurality of pixel circuits 11. The data lines D1 to DM provide data voltages for displaying image signals to the pixel circuits 11, and the scan lines S1 to SN provide select voltages for selecting the pixel circuits 11 to the pixel circuits 11. The pixel circuits 11 are formed at pixel areas defined by two neighboring data lines and two neighboring scan lines.
The scan driver 20 sequentially applies the select signals to the scan lines S1 to SN, and the data driver 30 applies the data voltages for displaying the image signals to the data lines D1 to DM.
Methods for driving the organic emitting cells through the pixel circuits 11 include a passive matrix method and an active matrix method which uses thin-film transistors (TFTs). The passive matrix method includes forming anodes and cathodes arranged perpendicular to each other, selecting lines, and driving the organic emitting cells. The active matrix method includes using the TFTs to select lines, storing data in capacitors of the pixels, and driving the organic emitting cells.
The active matrix method includes a voltage programming method and a current programming method according to patterns of signals applied to maintain the voltage at a capacitor. The current programming method supplies a data current for representing gray scales to pixel circuits to display the image, and the voltage programming method supplies a data voltage for representing gray scales to the pixel circuits to display the image.
The current programming method fails to obtain a charge time for charging loads of data lines since it controls an organic EL element by using a fine current, and accordingly, the voltage programming method is generally used.
Regardless of whether the programming methods are voltage or current, an organic EL element may emit light that is not intended because of undesired current during programming of the data in the pixel circuits, and, thus, normal black levels may not be properly represented.